The reason robots try to get accounts is to create link backs from high volume sites to lesser ranked web sites because Google takes into account the fact that higher volume sites are considered quality(for search validity) sites so if it links back to a lower ranked site, the value of the lower volume site is increased for organic search results. Getting a link back in a really high volume site can move the lesser site into the first page of search results.
Their assumption is that the link is an indication of how significant the site is that has the incoming link,
Just putting a link to a personal web site in the signature line of a forum in a mega site, like Trip Advisor or Amazon is worth a lot of money to the lesser site. It generates stronger position in the search results which translates into big differences in income.
In my field of tourism, fake referrals posted by small hotels or tour operators can mean up to $50,000 in increased sales for each well placed referral, positive review or link back so there is a very great incentive to cheat.